1. Field of the Invention
This invention relates to oil and gas production wells and particularly to methods for removing paraffin from such wells.
2. Description of the Prior Art
Paraffin formation in producing oil wells, and particularly those wells producing high-paraffin containing oils, significantly interferes with the effectiveness and efficiency of the well production. In severe cases, the presence of paraffin can, in essence, cause the oil production from a given well to cease.
Generally, the paraffin crystallizes out of and solidifies in the well bore, the area surrounding the well bore, the casing and tubing of the well thereby blocking further production.
In the past, attempts have been made to remove the paraffin by mechanically cutting it out of the well or using a so-called "hot-oiling" technique. The cutting procedure is a relatively crude procedure and requires extensive well shutdown time. Moreover, it is extremely inefficient inasmuch as substantial amounts of paraffin remain in the well. This remaining paraffin provides seed crystals which promote the rapid formation of additional paraffin. Consequently, paraffin builds up and the blocking situation quickly occurs again.
In the hot-oiling method, produced crude is heated to a temperature well above the melting point of the paraffin and is then circulated down through the annulus of the well and returned to a hot-oil heating truck via the production tubing. The purpose here is for the hot oil to melt and/or dissolve the paraffin so that it can be removed from the well in liquid form. This is an expensive method since the crude must be put through a heater treater along with a demulsifier in order to facilitate the removal of solids and water therefrom. In this method, the crude oil used is taken from the stock tank and has thus already made one pass through the treating facility and has already been demulsified. Another disadvantage to this method is that in many instances, 100% of the fluid injected is not recovered and thus some is lost to the reservoir.
During the hot-oiling process, a paraffin dispersant which is based on a petroleum sulfonate is added to the crude as it is being heated. The paraffin dispersant assists in dispersing the melted paraffin in the hot-oil phase.
Moreover, this technique is very dangerous, particularly with wells producing a crude having a low flash point. Indeed, such wells cannot be hot-oiled because the auto-ignition temperature of the oil is so low. Thus, bringing the oil in direct contact with a heating mechanism creates a substantial fire hazard.
Another procedure that has been tried is the so-called "hot acid" technique. In this process, an attempt is made to melt paraffin using a combination of hot water, heated xylene and hot acid. However, while this mixture may have some effect on the removal of carbonate scale build-up in wells, it has not presented a satisfactory answer to paraffin removal.
An additional disadvantage of each of the above methods is due to the fact that normally it is desirable to subject a well to acidization at some subsequent point in time after the paraffin removal. The mechanical cutting technique, hot-acid technique, and the hot-oil technique leave the well bore, the area surrounding the well bore, casing and tubing "oil wet". This is a disadvantageous situation for subsequent acidization. In the acidization technique, a mineral acid solution is introduced into the well to remove mineral deposits. The acid solution is aqueous and, if the interior portions and mechanical elements of the well are oil wet, direct contact of these surfaces with the acid solution is inhibited making the acidization treatment much less effective.
Such stimulation of oil and gas wells is a well-known process and is described in detail in U.S. Pat. No. 4,541,483, the contents of which are incorporated herein by reference.
Another problem associated with the prior art processes is that the paraffin which was recovered by mechanical means or by the hot-oil technique had to be dispersed of in a manner consistent with EPA standards of a particular state. This material was not usable for any purpose and represented an economic loss. In some cases, when an attempt to recover the paraffin from a liquid containing it, such as in the hot-oil technique, significant difficulties were encountered. Thus, if paraffin was treated in an attempt to separate it (from a water emulsion thereof), the actual emulsion pad, i.e., that layer of the actual emulsion separating the aqueous from the hydrocarbon layer was so wide as to disrupt the entire system. As a result, it was essentially impossible to separate the fluids from the emulsion.